Digg implements "dupe detection" technology
Digg is piloting a new duplicate content detection system. The technology was introduced with it's latest code release. The algorithm behind this technology is intelligent enough to analyze components from differing urls, and determine whether or not the content being submitted is identical to any content previously submitted to Digg. This ultimately provides them with "a reliable way of identifying identical content from the same source."
If you're interested in learning more about this feature, here's the Digg announcement:
Digg the Blog � Blog Archive � Digg: Dupe Detection Updates Are Here